1. Field of the Invention
The present invention relates to a method of controlling the operation of a furnace and, more particularly, to a control of combustion in a heating furnace which is used for reheating steel slabs prior to a hot rolling. Still more particularly, the invention is concerned with a method of controlling combustion in a heating furnace which method is adapted for furnace temperature control where operating condition of the furnace are widely varied.
Generally, in a heating furnace, steel slabs charged thereinto are transferred while they are heated up to reach the desired temperature and a good soaking condition before they leave the furnace. The steel slabs taken out of the furnace are delivered to the rolling line to be rolled in accordance with a predetermined rolling plan. Practically, the rolling condition is often changed in accordance with the actual state of the rolling. Usually, the change of the rolling condition is made by the operator in accordance with the signals delivered by a plurality of sensors. The change of rolling condition which is experienced most often is the change of rolling rate per unit time T/H (tons per hour) and pause or suspension of rolling in accordance with the working standard at the time of change of lot. Also, accidental stop of rolling is also included. When such a change of rolling condition is instructed by the operator at the control desk or given by a host computer, the state of operation of the furnace is affected more or less.
2. Description of the Prior Art
It is a common practice to accommodate the change of rolling condition by the change of operation state of the furnace. More specifically, the change of rolling condition is met by heating steel slabs to the aimed temperature as soon as possible and by increasing or decreasing the time length during which the slab is maintained at the aimed temperature. When the heat preservation time is prolonged, the whole part of the furnace including the slab inlet side is disadvantageously raised due to gas flow in the furnace to increase the loss of heat energy from the flue and furnace wall. This poses a serious problem from the view point of saving of energy, because the furnace consumes a considerably large amount of heat, but no effective countermeasure has been proposed yet.
U.S. Pat. No. 3,604,695, entitled "Method and Apparatus for Controlling a Slab Reheat Furnace" discloses a controlling method and apparatus for controlling a heating means in the heating zone of the furnace such that steel slabs leaving the heating zone has been heated to a predetermined temperature. More practically, the average temperature of each slab in the heating zone is predicted as a function of gas temperature in the heating zone, thermal characteristics and size of the slab, position of the slab in the furnace and moving speed and thermal hysteresis, and the heating means are controlled such that the predicted temperature coincides with the desired temperature in accordance with the temperature locus of the slab. It is, however, not preferred from the view point of saving of energy to apply the same furnace temperature locus when the operating conditions of the rolling mills and furnace are changed. The aforementioned U.S. Pat. No. 3,604,695 in no way discloses this point.